p53 is known as an important factor for inhibiting canceration of cells. p53 is a transcription factor that induces the expression of genes involved in the cell cycle and cellular apoptosis in response to various stresses. p53 is thought to inhibit canceration of cells by a transcription regulating function thereof. In fact, deletion or mutation of the p53 gene is observed in about half of human cancer cases.
Meanwhile, overexpression of murine double minute 2 (MDM2), a type of E3 ubiquitin ligase, is known as a factor for canceration of cells that are cancerated in spite of the presence of normal p53. MDM2 is a protein whose expression is induced by p53. MDM2 negatively regulates p53 by binding to the transcription activity domain of p53 to decrease the transcription activity of p53, exporting p53 out of the nucleus, and mediating degradation of p53 by acting as an ubiquitination ligase against p53. Therefore, it is thought that inactivation of functions of and degradation of p53 are promoted in cells in which MDM2 is overexpressed, resulting in canceration (Non Patent Document 1).
Paying attention to such functions of MDM2, many approaches have been attempted using substances that inhibit the suppression of p53 functions by MDM2 as candidate anti-tumor agents. Examples of MDM2 inhibitors targeting the MDM2-p53 binding site have been reported, which include spirooxindole derivatives (Patent Documents 1 to 15 and Non Patent Documents 1 to 3), indole derivatives (Patent Document 16), pyrrolidine-2-carboxamide derivatives (Patent Document 17), pyrrolidinone derivatives (Patent Document 18), isoindolinone derivatives (Patent Document 19 and Non Patent Document 4) and dispiropyrrolidine compounds (Patent Document 20).
FLT3 is a protein belonging to receptor tyrosine kinase class III together with KIT, FMS and PDGFR, etc., and is thought to be involved in the hematopoietic system (Non Patent Documents 5 to 8). Its structure has an extracellular region composed of five immunoglobulin-like domains, one juxtamembrane region (JM domain), two tyrosine kinase domains (TK1 and TK2) divided by a kinase insert domain (KI domain), and a C-terminal domain. FLT3 is highly expressed in brain, placenta, liver and hematopoietic stem cells (Non Patent Documents 6 to 9).
A ligand of FLT3 (FL) is expressed in stromal cells of bone marrow and stimulates stem cells, either alone or in collaboration with other cytokines (Non Patent Documents 10 to 13). The ligand-receptor interaction between FL and FLT3 is considered to have important functions in the hematopoietic system.
Meanwhile, high expression of FLT3 is observed in most cases in samples of acute myeloid leukemia (AML) or acute lymphatic leukemia (ALL) patients, and high expression of FLT3 is also seen in chronic myeloid leukemia (CML). It is also known that growth of AML cells is more remarkably enhanced than that of ALL cells by stimulation of FL (Non Patent Documents 14 to 18). The FLT3 gene is the gene that is most frequently mutated in acute myeloid leukemia (AML) cases, and either of internal tandem duplications (ITDs) in the juxtamembrane region (Non Patent Document 19) or a mutation in the FLT3 activation loop region (Non Patent Document 20) is confirmed in approximately 30% to 35% of patients. The mutation of FLT3-ITD or the activation loop region is associated with constitutive activation of tyrosine kinase activity.
N-(5-tert-Butyl-isoxazol-3-yl)-N′-{4-[7-(2-morpholin-4-yl-ethoxy)imidazo[2,1-b][1,3]benzothiazol-2-yl]phenyl}urea (quizartinib) having FLT3 inhibiting activity is known to have anti-tumor activity. Treatment of various cancers using quizartinib has been proposed in documents. Various dosing regimens have been reported. See, for example, Patent Documents 21 to 23 (which are incorporated herein by reference in their entirety). Also, the effects of combined use of quizartinib and anthracycline, a topoisomerase inhibitor or a tumor cell metabolic antagonist have been reported (Patent Document 24).
As for the relation between an MDM2 inhibitor and a FLT3 inhibitor, it has been reported that administration of an MDM2 inhibitor is preferred for patients whose cells contain FLT3 having an activating mutation (Patent Document 25). This document also states that combined administration of a FLT3 inhibitor and an MDM2 inhibitor is preferred for patients whose cells contain FLT3 having an activating mutation, but it does not disclose specific effects of the combined use of specific drugs.
There are various reports on the effects of combined use of various MDM2 inhibitors and various anti-tumor agents (Patent Documents 26 to 29).